On 2009-11-13, George wrote:

OK, thanks. Just to clarify: with collar ties (and suitably-sized
rafters), the ridge load would be much lower?

Consider a frame consisting of two walls studs and two rafters, with
hinges at all the connections. That configuration is not stable: it
can freely deform via a mechanism in which the ridge falls and the
tops of the walls spread apart. The two usual ways to deal with this
are either to provide a tension element (ceiling joist or collar tie)
to prevent the walls from spreading, or to provide a structural ridge
element which is properly supported.

I think it is typical to do just one or the other, so if you are using
collar ties, there would be no ridge load at all. But you could
design a hybrid solution that uses both collar ties and a structural
ridge. In that case, yes, the collar ties would reduce the load on
the ridge.

When you say the ridge is holding up the roof, does that mean their
rafters are in tension?

Keep in mind that the gravity load of the roof provides a distributed
axial force on the rafter, so whether the rafter is in compression or
tension can change along its length. When using a structural ridge,
the reaction from the ridge puts the top end of the rafter in
tensions, but the other loads are compressive, so the bottom end of
the rafter is in compression.

Cheers, Wayne